Cardiac pacemakers are implantable devices that replace or supplement a heart's compromised ability to pace itself (i.e., bradycardia) due to chronotropic incompetence or a conduction system defect by delivering electrical pacing pulses to the heart. Implantable cardioverter/defibrillators (ICD's) are devices that deliver electrical energy to the heart in order to reverse excessively rapid heart rates (tachycardia) including life threatening cardiac arrhythmias such as ventricular fibrillation. Since some patients have conditions that necessitate pacing and also render them vulnerable to life-threatening arrhythmias, implantable cardiac devices have been developed that combine both functions in a single device.
Most pacemakers today are operated in some sort of synchronous mode where the pacing pulses are delivered in a manner that is dependent upon the intrinsic depolarizations of the heart as sensed by the pacemaker. ICD's must also sense the electrical activity of the heart in order to detect an arrhythmia that will trigger delivery of the shock pulse in an attempt to reverse the condition. Such sensing information could be used to initiate another mode of therapy, and efforts have been made in the past to combine automatic drug delivery with either pacemakers, ICD's, or both to treat cardiac arrhythmias. U.S. Pat. No. 5,269,301, for example, deals with a system for delivering multi-modal therapy at the patient's bedside in order to treat heart conditions in which pacing, defibrillation, and drug delivery are under the control of a central processor. U.S. Pat. No. 5,087,243 discusses a drug delivery device incorporated into an ICD for delivering an antiarrhythmic drug iontophoretically directly to the myocardium via the patch electrodes used for electrical defibrillation upon detection of an arrhythmia. U.S. Pat. No. 5,893,881 describes an ICD with an integrated drug delivery device for delivery of a pain medication just prior to the delivery of a defibrillation shock. That patent also contemplates a separate implantable drug delivery device that communicates with the ICD via low-frequency radio waves transmitted through the body using a method described in U.S. Pat. No. 4,897,987.
Implantable drug delivery systems suffer from a number of disadvantages, however. Although the drug reservoir of such a device can be replenished, it is difficult to change the drug once it is put into the reservoir, making patient management difficult in cases where a patient's condition either changes or otherwise requires a change of medication. In addition, drugs degrade over time. Finally, there is the risk of leakage from the reservoir, the consequences of which can range from an annoyance to a medical emergency.